1. Technical Field of the Invention
The present invention discloses a method for the elimination of skew across a plurality of bit streams, as found in a Multiple Lane Distribution (MLD) system.
2. Background of the Invention
The International Telecommunication Union, Telecommunication Standardization Sector (ITU-T) Recommendation G.709, “Interfaces for the optical transport network (OTN)” defines the Optical Transport Network (OTN) frame as consisting of three main sections; the 16 byte overhead (OH) section; the 3808 byte payload (PL) section and the 256 byte forward error correction (FEC) section. The structure of the OTN frame is illustrated in FIG. 1. OTN is able to transport via a number of standard transmission rates, defined by Optical Data Unit (ODU) k. To date, transmission rates where k=1, 2 or 3 have been defined, while k=4 is currently under development.
The 16 byte OH section is comprised of the Frame Alignment Signal (FAS), the Optical Transport Unit (OTU) OH, the Optical Data Unit (ODU) OH and the Optical Payload Unit (OPU) OH, as illustrated in FIG 2. The FAS is located in row 1, columns 1-7 of the OTN frame, and includes the Multi-Frame Alignment Signal (MFAS). The OTU OH is located in row 1, columns 8-14, and is responsible for Section Monitoring (SM) and General Communication Channel 0 (GCC0) and row 1 of columns 13-14 contains bytes reserved for future international standardization (RES). The ODUk OH is located in rows 2-4, columns 1-14, and is responsible for Path Monitoring (PM) Tandem Connection Monitoring (TCM), General Communication Channels 1 and 2 (GCC1/GCC2) ODUk Automatic Protection Switching and Protection Communication Channel (APS/PCC), Fault Type and Fault Location Reporting Communication Channel (FTFL) ODUk Experimental Overhead (EXP) and additional ODUk OH bytes reserved for future international standardization (RES), the reserved OH bytes are located in row 2, columns 1-3, and row 4, columns 9-14. The OPUk OH is located in rows 1-4, columns 15-16, and is responsible for the Payload Structure Identifier (PSI) and the Payload Type (PT), as well as mapping and concatenation operations.
When transmitting a bit stream over any transmission network, such as the aforementioned Optical Transport Network (OTN), it may be impractical to transmit the bit stream serially due to technical or market constraints, such as the high costs associated with utilizing optical components for serial transmissions. Therefore, the single bit stream may be split into multiple bit streams through Multiple Lane Distribution (MLD), allowing the single bit stream to be transmitted in a parallel fashion. MLD encapsulates the data in frames for transmission and either manipulates the transmitted data for re-alignment or uses inherent patterns within the transmitted data for re-alignment.
After a bit stream has been transmitted as multiple bit streams through MLD, each of the multiple bit streams may be skewed, leaving the data unaligned at the receiver. An example of a skewed MLD system is shown in FIG. 3. As illustrated, serial bit stream 1 enters a 1:N de-multiplexer (DE-MUX) 4 located in transmitter 2. The DE-MUX 4 divides serial bit stream 1 into multiple bit streams 1(1)-1(n) through MLD. Each of bit streams 1(1)-1(n) are fed into transmitters 5(1)-5(n), respectively. Bit streams 1(1)-1(n) are then transmitted from the transmitter 2 into channels 6(1)-6(n), where the data may become skewed. Upon arrival at the receiver 3, the skewed bit streams 1(1)-1(n) are fed into framers 7(1)-7(n), respectively, which recognize inherent patterns in the transmitted data necessary for re-alignment or the manipulated data. Framers 7(1)-7(n) each write in 10 the data to buffers or First-In-First-Out (FIFO) buffers 8(1)-8(n). An aggregator 9 then reads out 11 the data from FIFOs 8(1)-8(n). The data may be skewed due to a number of external factors, such as: the data from multiple communication lanes, such as bit streams 1(1) and 1(n), travel at different times or speeds; or the data from framers 7(1)-7(n) are written into 10 the FIFOs 8(1)-8(n) at different times, thereby causing FIFOs 8(1)-8(n), which are cyclical in nature, to rotate out of synchronization. The latter is shown in FIG. 4, where upon a first clock cycle (CLK1), FIFO 8(1) location 0 is the first receive location, but upon a second clock cycle (CLK2), FIFO 8(1) location 0 has rotated to a new location, while FIFO 8(n) location 0 is still at the first receive location. Subsequently, when aggregator 9 of FIG. 3 reads out 11 the data from FIFOs 8(1)-8(n), this data may be skewed due to the differing time periods in which data from each of framers 7(1)-7(n) was written 10 into FIFOs 8(1)-8(n). Bit stream 34, now output from aggregator 9, contains skewed data.
In the “Prepublished Recommendation” G.798, “Characteristics of Optical Transport Network Hierarchy Equipment Functional Blocks,” the ITU-T defines an OTL loss of lane alignment defect (dLOL) as generated for multilane interfaces based on the state of the lane alignment process; if the multilane alignment process is in the out-of-alignment (OLA) state for a defined period of time, dLOL is declared. Therefore, a method of eliminating skew across a plurality of bit streams, including compensation for dLOL as found in an MLD system, is required.